Optical components are becoming increasingly more common in telecommunication networks. For example, waveguides such as optical fibers are used to carry information between different locations as optical signals. Such waveguides substantially confine the optical signals to propagation along a preferred path or paths. Similarly, other components such as sources, modulators, and converters often include guided regions that confine electromagnetic (EM) energy. Although metallic waveguides have a long history of use at longer wavelengths (e.g., microwaves), their usefulness as waveguides in the optical regime (e.g., 350 nm to 3 microns) is limited by their absorption. Thus, dielectric waveguiding regions are preferred in many optical applications.
One example of an optical component that confines electromagnetic energy is a fiber laser. Typically, such lasers include a high-index core that radially confines EM radiation through total internal reflection (TIR). In addition, they include refractive index modulations along their length to axially confine radiation and define a lasing cavity. For example, two Bragg gratings can surround a gain medium and define end reflectors, thereby forming what is called a “distributed Bragg reflector (DBR) laser.” Alternatively, the axial modulation can extend throughout the length of the gain medium to form a “distributed feedback (DFB) laser.”
One way of thinking about the resonant modes for such cavities is that they correspond to modes that spend a long time in the cavity. In other words, they correspond to modes at frequencies where the group velocity, vg=∂ω/∂β, approaches zero (where ω(β) gives the dispersion relation for a mode with angular frequency ω at a longitudinal wavevector β). This condition is equivalent to a divergence in the density of states (DOS) as a function of frequency ω.
Photolithographic techniques are typically used to form the axial index modulations in fiber lasers. For example, high-intensity regions of a standing wave illumination pattern can induce photo-refractive changes in index along the length of the fiber.